1. Field of the Invention
The present invention relates to light collection through the use of scintillators. In particular, the present invention relates to detecting the presence of energetic charged particles through their emission of scintillation light and the collection of that light.
2. Description of the Prior Art
A charged particle moving through an active media loses energy. Scintillator-based active media release part of these losses in the form of visible light. Most scintillators are made of inorganic crystals or organic plastics. The light, represented by the white line in FIG. 1, is bounced back and forth inside a scintillating cell to be either completely absorbed or to be registered by the photo detector if the photon scatters into it. Because the active area of the photo detector is usually significantly (100-1000 times) less than the cell area, to which the detector is directly coupled, the probability of registration is low. The area of the cell just in front of the photo detector is very visible to the photo detector. The farther area is less visible. As a result, the registered light yield across the cell area is non-uniform with respect to the particle impact position on the cell as illustrated in FIG. 2. If an air gap is introduced between the photo detector and the cell, the non-uniformity will be slightly less, but the light yield will be reduced rapidly with increasing gap size.
Present technology uses optical contact between an active media and photo detector or the addition of intervening media. Usually an optical glue or grease is used to improve light penetration from the active media to the photo detector, as shown in FIG. 1. Because the refractive index of the optical glue or grease is close to the refractive indexes of the active media and photo detector window, the light loss is less. While this increases the light collection efficiency in general, it does not reduce the non-uniformity in the registered light yield.
D'Ascenzo, et al. describes a highly granular hadronic calorimeter that consists of finely segmented arrays of plastic scintillators. A Geiger mode avalanche photodiode directly reads photons generated by the blue scintillators. For the direct coupling readout, the scintillating cell was totally wrapped in a Super-radiant VM2000 foil from 3M. To connect the photo detector, a window of 3×3 mm2 was open in the scintillator reflective wrapping. The photo detector plastic coverage that protects the active area was in contact with the scintillator. No special optical coupling was used. The uniformity of the light collection with respect to the particle impact position on the cell was not studied but it can be surmised that since the surface that the light hits is flat, it will suffer from the deficiencies described above.
Danilov describes the large-scale use of silicon photomultipliers (SiPMs) as photo detectors for a plastic scintillator-based hadron calorimeter. Each scintillator tile includes a wavelength-shifting fiber inserted in a groove on the tile. An air gap separates the SiPMs from the scintillators. The SiPMs are connected to one of the fiber ends while the other end of the fiber is covered with a mirror in order to increase light yield.
The response uniformity for a square shaped scintillating cell of 9 cm2 area and 3 mm thickness was studied for two cases and shown in FIG. 5 in Danilov. In the first case, the Kurary 1 mm outer diameter wavelength shifting fiber was glued into a 2 mm deep groove made diagonally in the cell. The photo detector with 1 mm2 active area was also glued to the fiber end. The uniformity of response across the cell area was almost flat with about a 25% slope along the wavelength shifting fiber direction (from corner to corner). In the second case, the cell was directly read out with the photo detector that had a 2.1×2.1 mm2 active area. The uniformity response of the cell with respect to the impact position of the charged particle was significantly worse than with the fiber readout.
There is a need for a more uniform method of light collection when using direct (i.e. fiberless) coupling.